The present invention relates to video key processing, and more particularly to a digital memory delay line for a video border generator that produces a widened key signal for border or shadow effects about a title video signal.
A video border generator provides a border or a shadow for a video image signal. To produce such a border or shadow a widened key signal is generated from an input key signal associated with the video image signal. The input key signal is subtracted from the widened key signal to produce a fill key signal that permits the addition of a border or shadow video fill signal to the video image signal. One such video border generator is incorporated into the Model 100 Video Switcher manufactured by The Grass Valley Group, Inc. of Grass Valley, Calif., United States of America. The input key signal is input to an input buffer amplifier, and then applied to a series of delay lines, each delay line having a delay equal to one horizontal video line. The outputs of the delay lines together with the input key signal are input to a combinational logic and output circuit to produce the widened key signal according to the desired mode determined by a command signal. These delay lines are analog glass delay lines that require complementary inputs. The delay lines produce a bipolar doublet output that is delayed from the input by approximately one horizontal line (1H). The doublet is compared with two thresholds, setting respective flip-flops to form a pulse identical with the key input timing. An additional delay line is used having taps to finely adjust the 1H delay for each glass delay line. This analog system is expensive and subject to errors typical of analog devices.
What is desired is a device for producing a precise 1H and 2H delay of an input key signal for a border/shadow generator without the expense of analog delay devices with their attendant errors.